Welded joint, method for manufacturing welded joint, and automotive component

Abstract

This welded joint is obtained by stacking and welding two or more steel materials and includes a weld metal, a heat-affected part, and a steel material part. The steel material part has, on the surface thereof, a zinc-based plating layer that contains 85 mass% or more of zinc and has an average thickness of 3-30 μm. A ZnO compound is deposited to the welding back face along the heat-affected part. The proportion of the length of the ZnO compound in the welding direction with respect to the length of the heat-affected part in the welding direction of the welding metal is more than 0% but not more than 80%.

Description

Welded joint, welded joint manufacturing method, and automobile part

[0001] The present disclosure relates to a welded joint, a method for manufacturing a welded joint, and an automotive part.

[0002] With climate change becoming a major social issue, the automotive industry is being called upon to improve fuel economy and electricity consumption by reducing the weight of vehicle bodies. From the perspective of steel materials, it is possible to reduce the weight of conventional vehicle bodies by increasing the strength and reducing the thickness of the steel sheets used. However, reducing the weight of automotive parts such as lower arms and subframes (chassis components), which require long-term durability, is not easy. Therefore, improving the corrosion resistance of welds is important from the perspective of achieving weight reduction in these automotive parts. The corrosion resistance of welds in the automotive industry is generally achieved by imparting rust prevention properties through chemical conversion coating and electrodeposition coating, but the application of zinc-plated materials is one method of further improving corrosion resistance.

[0003] For example, Japanese Patent Application Laid-Open No. 2017-187260 discloses a surface treatment method for zinc-plated steel material, which includes the steps of preparing a zinc-plated steel material having a base material and a zinc-containing plating layer formed on the surface of the base material, and removing or altering a portion of the plating layer by heating with a flame generated by burning a combustible gas, wherein the combustible gas used in the step of removing or altering a portion of the plating layer is a gas containing 25% by volume or more and 44% by volume or less of ethylene, with the remainder being hydrogen and unavoidable impurities.

[0004] Furthermore, Japanese Patent Application Laid-Open No. 2005-40806 discloses a laser irradiation arc welding method in which the arc welding method is a consumable electrode pulse arc welding method, the laser irradiation position is adjusted to be near the boundary between the molten part and the non-molten part in front of the molten pool, the focused diameter of the laser irradiation part is set to a diameter slightly larger than the gap length existing in the weld joint, and the energy density of the laser irradiation part is set to a value that evaporates the zinc plating of the irradiated part.

[0005] Furthermore, Japanese Patent Application Laid-Open No. 2007-38241 discloses a method for welding zinc-based plated steel sheets, in which a resin coating layer is formed on the back surface of a portion to be welded of a zinc-based plated steel sheet, and then the portion to be welded is arc-welded or laser-welded from the front surface side.

[0006] Furthermore, Japanese Patent Application Laid-Open No. 2004-1009 discloses a method for welding zinc-based plated steel sheets, in which a cooling gas supply container with an open top is placed in close contact with the back side of the welding location of a zinc-based plated steel sheet to be welded, and the welding location is arc-welded while cooling gas 7 is introduced through a gas inlet provided on one side of the container and the cooling gas is discharged from a gas outlet provided on the other side.

[0007] Welded joints used in automobile parts and the like are required to have corrosion resistance at the weld. For example, in order to improve the corrosion resistance of the weld, rust prevention is imparted by a coating film (e.g., electrodeposition coating) having rust prevention properties. It is also effective to use a steel material having a zinc-based plating layer as the base material. However, heat input during welding causes evaporation of the zinc-based plating layer, generating ZnO compounds, which can adhere to the backside of the weld. These ZnO compounds hinder the formation of a coating film and reduce corrosion resistance.

[0008] In view of the above circumstances, an object of the present disclosure is to provide a welded joint that has excellent corrosion resistance after painting, a method for manufacturing the welded joint, and an automotive part that has the welded joint.

[0009] Means for solving the problems include the following aspects. <1> A welded joint formed by welding two or more overlapping steel materials, the welded joint having a weld metal, a heat-affected zone, and a steel material portion, the steel material portion having a surface with a zinc-based plating layer containing 85 mass% or more of zinc and having an average thickness of 3 μm to 30 μm, a ZnO compound adhering along the heat-affected zone on the back side of the weld, and a ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction of the weld metal is more than 0% and 80% or less. <2> The welded joint according to <1>, in which a ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction of the weld metal is 10% or more and 60% or less. <3> The welded joint according to <1> or <2>, having the ZnO compound, a chemical conversion coating, and an electrodeposition coating on at least the surface of the heat-affected zone. <4> A method for manufacturing a welded joint, comprising a welding step of manufacturing a welded joint by overlapping and welding two or more steel materials each having a zinc-based plating layer containing 85 mass% or more of zinc and having an average thickness of 3 μm or more and 30 μm or less, wherein in the welding step, the welding is performed while spraying at least one fluid selected from the group consisting of gases and liquids at a flow rate of 0.3 m / s or more and 14.0 m / s or less onto a portion of the back surface of the weld that will become the heat-affected zone. <5> The method for manufacturing a welded joint according to <4>, wherein the fluid is an inert gas. <6> An automotive part having the welded joint according to any one of <1> to <3>.

[0010] According to the present disclosure, there are provided a welded joint having excellent corrosion resistance after painting, a method for manufacturing the welded joint, and an automobile part having the welded joint.

[0011] Fig. 1 is a plan view showing the weld back surface of a welded joint according to an embodiment of the present disclosure; Fig. 2 is a plan view showing the weld back surface of a conventional welded joint; Fig. 3 is a schematic cross-sectional view showing a lap welded joint as a welded joint according to an embodiment of the present disclosure; Fig. 4 is a schematic cross-sectional view showing a lap fillet welded joint as a welded joint according to an embodiment of the present disclosure; Fig. 5 is a schematic cross-sectional view showing a T-fillet welded joint as a welded joint according to an embodiment of the present disclosure; Fig. 6 is a schematic cross-sectional view for explaining a method for manufacturing a welded joint according to an embodiment of the present disclosure.

[0012] An embodiment that is an example of the present disclosure will be described. These descriptions and examples are illustrative of the embodiments and do not limit the scope of the invention. In this specification, when a numerical range expressed using "to" is used, unless the numerical values ​​before and after "to" are followed by "greater than" or "less than," it means a range that includes the numerical values ​​as the lower and upper limits. Furthermore, when the numerical values ​​before and after "to" are followed by "greater than" or "less than," it means a range that does not include the numerical values ​​as the lower and upper limits. In the numerical ranges described in stages in this specification, the upper limit of a certain numerical range may be replaced by the upper limit of another numerical range described in stages, or may be replaced by a value shown in an example. Furthermore, the lower limit of a certain numerical range may be replaced by the lower limit of another numerical range described in stages, or may be replaced by a value shown in an example. Furthermore, "%" in the content refers to "% by mass" unless otherwise specified. A content (%) of "0 to" means that the component is an optional component and may not be present.

[0013] Each component may contain multiple corresponding substances. When referring to the amount of each component in a composition, if multiple substances corresponding to each component are present in the composition, the total amount of those multiple substances present in the composition is meant unless otherwise specified. The term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes as long as the intended effect of the process is achieved.

[0014] <Welded joint> A welded joint according to an embodiment of the present disclosure is a welded joint formed by welding two or more overlapping steel materials. The welded joint has a weld metal, a heat-affected zone, and a steel material portion. The steel material portion has a zinc-based plating layer on its surface, containing 85 mass% or more of zinc and having an average thickness of 3 μm or more and 30 μm or less. The welded joint has ZnO compounds adhered along the heat-affected zone on the back side of the weld. The ratio of the length of the ZnO compounds in the welding direction to the length of the heat-affected zone of the weld metal in the welding direction (hereinafter also referred to as the "ZnO length ratio") is greater than 0% and less than 80%.

[0015] The welded joint according to the embodiment of the present disclosure has the above-described configuration, and is therefore a welded joint with excellent corrosion resistance after painting. The welded joint according to the present disclosure was discovered based on the following findings.

[0016] Welded joints used in automobile parts and the like require corrosion resistance at the welds. For example, in automobile parts, a coating film with rust prevention properties is formed on the surface to improve the corrosion resistance of the welds. Specifically, this is achieved by chemical conversion coating and electrodeposition coating. Another method for further improving corrosion resistance is the application of steel materials with a zinc-based plating layer (galvanized material). The use of zinc-plated materials effectively prevents iron corrosion through the sacrificial corrosion protection of zinc. However, heat input during welding can cause evaporation of the zinc-based plating layer, which can actually reduce the corrosion resistance of the welds. Specifically, zinc evaporated from the zinc-based plating layer reacts with oxygen in the air and burns, generating ZnO compounds along the heat-affected zone. These ZnO compounds hinder the formation of coating films (e.g., chemical conversion coatings and electrodeposition coatings), making the area prone to corrosion, i.e., areas with low corrosion resistance.

[0017] In contrast, in the welded joint according to the embodiment of the present disclosure, ZnO compounds are deposited along the heat-affected zone on the reverse side of the weld, and the length of these ZnO compounds in the welding direction relative to the length of the heat-affected zone in the welding direction (ZnO length ratio) is greater than 0% and less than or equal to 80%. Thus, in the welded joint according to the embodiment of the present disclosure, the amount of ZnO compounds deposited along the heat-affected zone is reduced. Therefore, the formation of a coating film (e.g., the formation of a chemical conversion coating and an electrodeposition coating film) in the heat-affected zone is prevented from being hindered, resulting in a welded joint with excellent corrosion resistance after painting.

[0018] As a result, the welded joint according to the embodiment of the present disclosure has excellent corrosion resistance after painting. Furthermore, automotive parts (e.g., chassis components) having this welded joint are expected to have a longer life. These effects improve the corrosion resistance of automotive parts after painting and also promote weight reduction of the vehicle body.

[0019] In addition, a method for suppressing the amount of ZnO compounds generated along the heat-affected zone on the back surface of the weld in the welded joint according to an embodiment of the present disclosure, specifically a method for controlling the ZnO length ratio to be within the above-mentioned range, will be described in detail later.

[0020] Next, a welded joint according to an embodiment of the present disclosure will be described in detail.

[0021] ZnO Compounds In the welded joint according to the embodiment of the present disclosure, ZnO compounds are adhered along the heat-affected zone on the back surface of the weld.

[0022] As an example of a conventional welded joint, a plan view showing the weld back surface of a conventional welded joint is shown in Figure 2. As shown in Figure 2, a conventional welded joint 100 has a base metal 2 and a heat-affected zone 4 that is long in the welding direction X and is formed in the base metal 2. ZnO compounds 60 are deposited along the heat-affected zone 4. In this conventional welded joint 100, the length L of the weld metal in the welding direction X is Ｗ is 100 mm, and the length L of the ZnO compound 60 in the welding direction X on one side Ｚ１０ is 100 mm, and the length L of the ZnO compound 60 in the welding direction X on the other side Ｚ２０ The length L of the weld metal in the welding direction X is 100 mm. Ｗ The length of the ZnO compound 60 in the welding direction X ((L Ｚ１０ +L Ｚ２０ ) / 2) ratio (((L Ｚ１０ +L Ｚ２０ ) / 2) / L Ｗ ×100) is 100%.

[0023] Next, an example of a welded joint according to an embodiment of the present disclosure is shown in FIG. 1. FIG. 1 is a plan view showing the welded back surface of a welded joint according to an embodiment of the present disclosure. The welded joint 10 shown in FIG. 1 has a base material 2 and a heat-affected zone 4 that is long in the welding direction X and is formed in the base material 2. The area of ​​the base material 2 other than the heat-affected zone 4 is a steel part. ZnO compounds 6 are adhered along the heat-affected zone 4. However, in this welded joint 10, the amount of ZnO compounds 6 that are adhered along the heat-affected zone 4 is reduced. In the welded joint 10, the length L of the weld metal in the welding direction X is Ｗis 100 mm, and the length L in the welding direction X on one side of the ZnO compound 6 Ｚ１ The length L in the welding direction X on the other side of the ZnO compound 6 is 25 mm. Ｚ２ The length L of the weld metal in the welding direction X is 50 mm. Ｗ The length of the ZnO compound 6 in the welding direction X ((L Ｚ１ +L Ｚ２ ) / 2) ratio (((L Ｚ１ +L Ｚ２ ) / 2) / L Ｗ × 100) is 37.5%.

[0024] Thus, in the welded joint according to the embodiment of the present disclosure, ZnO compounds are adhered along the heat-affected zone on the back surface of the weld, and the length of these ZnO compounds in the welding direction relative to the length of the heat-affected zone in the welding direction (ZnO length ratio) is more than 0% and not more than 80%. This results in excellent corrosion resistance after painting. From the viewpoint of corrosion resistance after painting, the ZnO length ratio is preferably not more than 60%, and more preferably not more than 50%. On the other hand, from the viewpoint of easily suppressing the generation of ZnO compounds, the lower limit of the ZnO length ratio may be 10% or more, or may be 20% or more.

[0025] The ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction (ZnO length ratio) is the length L of the weld metal in the welding direction X. Ｗ and the length L in the welding direction X on one side of the ZnO compound Ｚ１ and the length L in the welding direction X on the other side of the ZnO compound Ｚ２ and can be calculated by the following formula: ZnO length ratio = (((L Ｚ１ +L Ｚ２ ) / 2) / L Ｗ x100)

[0026] On the reverse side of the weld, if the ZnO compounds adhering along the heat-affected zone are continuous in the welding direction X (i.e., uninterrupted), the length of the continuous ZnO compounds in the welding direction is measured. If the ZnO compounds adhering along the heat-affected zone are interrupted in the welding direction X (i.e., in the form of two or more clumps), the length of each clump of ZnO compounds in the welding direction is measured, and the total value is taken as the length of the ZnO compounds. Furthermore, even if the weld line has a curved portion (i.e., a portion with a curvature), the method for measuring the length of the heat-affected zone in the welding direction and the length of the ZnO compounds in the welding direction is the same. In other words, even at the curved portion of the weld line, the length of the heat-affected zone in the welding direction and the length of the ZnO compounds in the welding direction are measured to determine the ZnO length ratio.

[0027] Whether or not ZnO compounds are attached along the heat-affected zone on the reverse side of the weld can be confirmed by measuring a cross section perpendicular to the welding direction with an electron probe microanalyzer (EPMA).

[0028] Here, a method for identifying the heat-affected zone will be described. The heat-affected zone generally refers to an area resulting from the transformation of the metal structure of the steel material as a result of heat input to the steel material by welding. However, the boundary between the heat-affected zone and its surroundings (i.e., the steel material portion that is not the heat-affected zone) is not easily identifiable by visual inspection, microscopic observation, or the like. The range set based on the length of the weld metal on the front weld surface of the welded joint can be considered to roughly correspond to the range of the original heat-affected zone. Furthermore, the ZnO compounds adhere along the heat-affected zone, and the range of the heat-affected zone set based on the length of the weld metal is considered to be an appropriate range for the adhesion of ZnO compounds. For the above reasons, the "length of the heat-affected zone" in this specification refers to the range 5 mm extended in the longitudinal direction from the length of the weld metal on the front weld surface of the welded joint.

[0029] Zinc-based plating layer The welded joint according to the embodiment of the present disclosure has, at least on the surface of the steel portion (i.e., the region other than the heat-affected zone in the base material), a zinc-based plating layer containing 85 mass% or more of zinc and having an average thickness of 3 μm or more and 30 μm or less.

[0030] Here, a method for measuring the zinc content in the plating layer will be described. First, a sample is embedded in resin and polished so that the cross section of the plating can be observed. After polishing, a scanning electron microscope (SEM) is used to perform SEM-EDS (Energy Dispersive X-ray Spectroscopy) to quantify the composition ratio of various elements (Zn, Al, Mg, Fe, Cr, Ni, Ti, etc.) contained in the plating, thereby determining the zinc content. As for the measurement locations, five cross sections of the plating in the non-heat-affected zone can be analyzed.

[0031] Examples of zinc-based plating layers containing 85% by mass or more of zinc include a GA plating (galvannealed) layer and a GI plating (hot-dip galvanized) layer. The GI plating layer can be formed, for example, by a plating process using a reduction furnace. The GA plating layer is formed by forming a hot-dip galvanized layer (GI plating layer) on a steel sheet (base material) and then performing an alloying process.

[0032] The zinc-based plating layer contains 85 mass % or more of zinc, and other components are not particularly limited. However, the GI plating layer and the GA plating layer preferably have the following compositions, respectively.

[0033] The GI plating layer may have a composition consisting of zinc, or may contain small amounts of other metal elements or impurities (e.g., cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc.) in addition to zinc. It may also contain inorganic substances such as silica, alumina, and titania. A preferred composition of the GI plating layer is, for example, a plating layer containing, by mass %, 0.01 to 0.20% Al, with the balance being Zn and impurities.

[0034] The GA plating layer may have a composition, for example, of a zinc-based alloy plating layer composed of zinc and another metal (e.g., at least one selected from the group consisting of iron, aluminum, cobalt, tin, nickel, chromium, titanium, magnesium, and manganese). In addition to these, the GA plating layer may contain small amounts of other metal elements or impurities (e.g., cobalt, molybdenum, tungsten, nickel, titanium, chromium, aluminum, manganese, iron, magnesium, lead, bismuth, antimony, tin, copper, cadmium, arsenic, etc.). Furthermore, the GA plating layer may further contain inorganic substances such as silica, alumina, and titania. A preferred composition of the GA plating layer is, for example, in mass %, 5 to 20% Fe and 0.01 to 0.20% Al, with the balance being Zn and impurities.

[0035] The average thickness of the zinc-based plating layer is 3 μm or more and 30 μm or less. If the average thickness is 3 μm or more, ZnO compounds are likely to be generated due to zinc evaporation during welding. However, in the welded joint according to the embodiment of the present disclosure, the ZnO length ratio is within the aforementioned range, and therefore the welded joint has excellent corrosion resistance after painting. On the other hand, if the average thickness is 30 μm or less, excessive evaporation of zinc during welding is suppressed, improving corrosion resistance after painting. The average thickness of the zinc-based plating layer is preferably 5 μm or more and 25 μm or less, and more preferably 10 μm or more and 20 μm or less. The average thickness of the zinc-based plating layer is the arithmetic mean value of plating thicknesses measured in 10 fields of view in an electron microscope image of the cross section of the steel material.

[0036] Coating: The welded joint according to the embodiment of the present disclosure has excellent corrosion resistance after painting. That is, to improve the corrosion resistance of the welded portion, a coating is formed on at least the surface of the heat-affected zone of the welded joint. Furthermore, a coating may be formed on the entire surface of the welded joint in addition to the heat-affected zone.

[0037] For the welded joint according to the embodiment of the present disclosure, an electrodeposition coating is preferred as the coating formed at least on the surface of the heat-affected zone from the viewpoint of corrosion resistance of the weld. Known electrodeposition coatings may be used, and either anionic or cationic electrodeposition coatings may be used. However, cationic electrodeposition coatings are preferred from the viewpoint of workability. Examples of electrodeposition coatings include those formed by electrodeposition coating using an aqueous paint containing a resin, a curing agent, and other additives. Examples of resins include aqueous resins (e.g., known aqueous resins such as acrylic resins, polyester resins, alkyd resins, epoxy resins, and polyurethane resins) having hydrophilic groups such as carboxyl groups, hydroxyl groups, methylol groups, amino groups, sulfonic acid groups, and polyoxyethylene bonds, and functional groups such as hydroxyl groups that react with curing agents. Examples of curing agents include melamine resins and blocked polyisocyanates. Other additives include known additives such as coloring pigments, optical interference pigments, extender pigments, dispersants, anti-settling agents, reaction accelerators, antifoaming agents, thickeners, rust inhibitors, ultraviolet absorbers, and surface conditioners.

[0038] The average thickness of the electrodeposition coating is preferably 1 to 40 μm, more preferably 5 to 30 μm.

[0039] When a coating film (e.g., an electrodeposition coating film) is formed on the surface of a welded joint, a chemical conversion coating may be formed between the welded joint and the coating film. That is, the coating film (e.g., an electrodeposition coating film) may be formed on at least the surface of the heat-affected zone via a chemical conversion coating film, or the coating film (e.g., an electrodeposition coating film) may be formed on the entire surface of the welded joint in addition to the heat-affected zone via a chemical conversion coating film.

[0040] Shape of the welded joint The welded joint according to the embodiment of the present disclosure is a welded joint formed by overlapping and welding two or more steel materials. Here, "lap welding" means that the steel materials are partially overlapped (for example, two steel plates are partially stacked) and welded. Examples of such welded joints include lap welded joints, lap fillet welded joints, and T-fillet welded joints.

[0041] A lap welded joint is a joint formed by stacking the ends of two steel sheets 21A and 22A, each having a zinc-based plating layer 8 on its surface, together and welding the stacked region from one side (the upper side in FIG. 3 ), as shown in FIG. 3 . In the lap welded joint shown in FIG. 3 , a heat-affected zone 4A is formed around the weld metal 3A, and the back side (the lower side in FIG. 3 ) of the welded side corresponds to the weld back side Y. ZnO compounds 6A adhere to the weld back side Y. A lap fillet welded joint is a joint formed by stacking the ends of two steel sheets 21B and 22B, each having a zinc-based plating layer 8 on its surface, together and welding the end face of one of the stacked steel sheets 21B to the flat surface (the upper flat surface in FIG. 3 ) of the other steel sheet 22B, as shown in FIG. 4 . In the lap fillet welded joint shown in FIG. 4 , a heat-affected zone 4B is formed around the weld metal 3B, and the back side (the lower side in FIG. 4 ) of the welded side corresponds to the weld back side Y. ZnO compounds 6B adhere to the weld back surface Y. In a lap fillet weld joint, the end surface of the other steel plate 22B may be further welded to the flat surface (the lower flat surface in FIG. 3 ) of one steel plate 21B. A T-shaped fillet weld joint is a joint formed by using steel plates 21 and 22C, each having a zinc-based plating layer 8 on its surface, and bringing the steel plate 21C into contact with the steel plate 22C in a vertical position, with the end surface of the steel plate 21C abutting the flat surface (the upper flat surface in FIG. 5 ) of the steel plate 22C, forming a T-shaped joint, as shown in FIG. 5 . In this T-shaped fillet weld joint, one side of the contact point between the end surface of the steel plate 21C and the flat surface (the upper flat surface in FIG. 3 ) of the steel plate 22C is welded. In the T-shaped fillet weld joint shown in FIG. 5 , a heat-affected zone 4C is formed around the weld metal 3C, and the back side (the lower side in FIG. 5 ) of the welded side becomes the weld back surface YA and YB. ZnO compounds 6CA and 6CB are attached to the weld back surfaces YA and YB, respectively, but the ZnO compound 6CA attached to the weld back surface YA is the object for measuring the length of the ZnO compound in the welding direction. In other words, the ZnO compound 6CA attached to the flat surface of steel plate 22C (the lower flat surface in FIG. 5) is the object of measurement, and the ZnO compound 6CB attached to the contact point between the end surface of steel plate 21C and the flat surface of steel plate 22C (the upper flat surface in FIG. 5) is not the object of measurement.

[0042] <Method for manufacturing a welded joint> Next, a method for manufacturing a welded joint according to an embodiment of the present disclosure will be described. Note that the method for manufacturing a welded joint according to an embodiment of the present disclosure can obtain the welded joint according to the embodiment of the present disclosure described above. In other words, the method for manufacturing a welded joint according to an embodiment of the present disclosure can produce a welded joint that has excellent corrosion resistance after painting.

[0043] A method for manufacturing a welded joint according to an embodiment of the present disclosure includes a welding step of manufacturing a welded joint by overlapping and welding two or more steel materials each having a zinc-based plating layer containing 85 mass% or more of zinc and having an average thickness of 3 μm or more and 30 μm or less, and in the welding step, welding is performed while spraying at least one fluid selected from the group consisting of gas and liquid at a flow rate of 0.3 m / s or more and 14.0 m / s or less onto a portion of the back surface of the weld that will become the heat-affected zone.

[0044] Here, a method for obtaining a welded joint according to the above-described embodiment of the present disclosure will be described, that is, a method for obtaining a welded joint in which the ratio of the length of the ZnO compounds in the welding direction to the length of the heat-affected zone in the welding direction (ZnO length ratio) is within the above-described range by suppressing the generation of ZnO compounds on the back surface of the weld.

[0045] The present inventors observed the formation behavior of ZnO compounds on the welded backside of galvanized materials using a high-speed camera and obtained the following findings. First, ZnO compounds were not generated depending on the temperature of the welded backside, but only galvanized vapor was generated. Furthermore, it was confirmed that ZnO compounds were not generated simply by the evaporation of galvanized coating, but rather that ZnO compounds were generated and adhered to the welded backside when zinc vapor burned. The combustion phenomenon occurred when galvanized vapor came into contact with the high-temperature portion of the welded backside, and it was confirmed that the combustion spread from the outermost layer that came into contact. Although manual wiping was performed to remove the adhered ZnO compounds, cross-sectional observation revealed that approximately 3 μm to 5 μm of ZnO compounds remained, even after wiping, and complete removal was not easy. Furthermore, in areas where ZnO compounds were attached, coating films such as electrodeposition coatings tended to fail to form or to become thin, which is thought to be the starting point for corrosion.

[0046] Combustion does not occur unless the following three elements are present: a combustible material, an oxygen supplier, and an ignition source. Based on the above findings, it is believed that adhesion of ZnO compounds can be reduced by performing welding in a situation where the following three elements are not present: zinc plating vapor (powder form) as a combustible material, oxygen in the air as an oxygen supplier, and the high-temperature part (heat-affected zone) on the reverse side of the weld as an ignition source.

[0047] Therefore, in the method for manufacturing a welded joint according to an embodiment of the present disclosure, welding is performed while spraying at least one fluid selected from the group consisting of gases and liquids onto the heat-affected zone on the back surface of the weld. This reduces the density of the zinc plating vapor, reduces heat conduction from the outermost layer of the back surface of the weld to the steam atmosphere, and suppresses combustion of the zinc vapor, thereby reducing the adhesion of ZnO compounds.

[0048] It is believed that ZnO compounds adhering to the weld back surface can be removed by a robust removal process such as shot blasting. However, the removal process of ZnO compounds can be difficult depending on the shape of the component. Specifically, it is difficult to perform the removal process on components whose weld back surface faces a closed space (enclosed space). Furthermore, performing the removal process also reduces production efficiency. Therefore, performing the removal process is undesirable. In contrast, the method for manufacturing a welded joint according to an embodiment of the present disclosure can reduce the adhesion of ZnO compounds in the first place, so the removal process of ZnO compounds is not necessary. Therefore, according to the method for manufacturing a welded joint according to an embodiment of the present disclosure, the welded joint according to the embodiment of the present disclosure can be obtained without performing the removal process. In other words, a method for manufacturing a welded joint according to an embodiment of the present disclosure, in which the welded joint does not have any traces of the removal process on the weld back surface, can be obtained.

[0049] Here, an example of a method for manufacturing a welded joint according to an embodiment of the present disclosure will be described with reference to the drawings. Fig. 6 is a schematic cross-sectional view for explaining a method for manufacturing a welded joint according to an embodiment of the present disclosure.

[0050] A method for manufacturing a welded joint according to an embodiment of the present disclosure first includes a welding step of overlapping (stacking) end portions of two steel sheets 21B and 22B, each having a zinc-based plating layer 6 containing 85 mass% or more of zinc and having an average thickness of 3 μm or more and 30 μm or less, and welding the end face of one of the stacked steel sheets 21B to the flat surface (the upper flat surface in FIG. 6) of the other steel sheet 22B to form a weld metal 3B, as shown in Fig. 6. Then, in this welding step, welding is performed while spraying gas Z (for example, by spraying gas using a nozzle 30 or the like) against a portion of the weld back surface Y that will become the heat-affected zone 4B.

[0051] In the welding process, the flow velocity of at least one fluid selected from the group consisting of gases and liquids sprayed onto the heat-affected zone on the back surface of the weld is set to 0.3 m / s or more and 14.0 m / s or less. Setting the flow velocity to 0.3 m / s or more reduces the density of the zinc plating vapor, suppressing combustion and reducing the adhesion of ZnO compounds. Setting the flow velocity to 14.0 m / s or less prevents gas leakage from the overlapping portion of the steel materials and prevents porosity defects from occurring inside the weld metal. The flow velocity is preferably set to 0.5 m / s or more and 12.0 m / s or less, more preferably 1.0 m / s or more and 10.0 m / s or less, and even more preferably 2.0 m / s or more and 8.0 m / s or less.

[0052] The flow velocity of the fluid is measured using a vane-type anemometer (product number EA739AF-21, manufactured by AS ONE). The tip of the vane probe for measuring the flow velocity is fixed at a position on the back side of the steel material that does not come into contact with the heat-affected zone (a position 20 mm vertically away from the heat-affected zone), and the average value measured for 10 seconds is defined as the flow velocity.

[0053] The fluid to be sprayed may be a gas or a liquid. Gases include air, inert gases (e.g., Ar), CO ２Examples of the liquid include rare gases such as oxygen, oxygen, nitrogen, helium, and neon, as well as mixtures of these. Since oxygen is easily combustible, when oxygen is included, its content is preferably equal to or lower than that of atmospheric air, and more preferably equal to or lower than 10% oxygen. Examples of the liquid include water, silicone oil, lubricating oil, and mixtures of these, or mixtures containing compressed air. When spraying the liquid, it is preferable to spray it as a spray liquid. Among these, the fluid is preferably an inert gas, from the viewpoint of suppressing the generation of ZnO compounds (i.e., the reaction between Zn and O) and reducing the adhesion of ZnO compounds.

[0054] In the present disclosure, when the steel material (base material) is a steel plate, the plate thickness is not particularly limited, but can be, for example, 1 mm or more and 30 mm or less.

[0055] For example, a weld joint according to an embodiment of the present disclosure can be obtained by gas-shielded arc welding of steel materials using a solid wire. In this case, the chemical composition of the weld metal includes components derived from the solid wire, which is the welding material, and the steel material, which is the base material.

[0056] (Automobile Parts) The welded joints according to the embodiments of the present disclosure can be used, for example, as automobile parts. Examples of automobile parts having the welded joints according to the embodiments of the present disclosure include subframes, lower arms, upper arms, and axle beams of chassis members.

[0057] Next, the feasibility and effects of the present disclosure will be explained in more detail using examples and comparative examples. However, the following examples do not limit the present disclosure, and any design changes that are made in accordance with the above and below spirit are included in the technical scope of the present disclosure.

[0058] The welded joints were obtained by gas shielded arc welding using a solid wire.

[0059] (Steel Sheet) A steel sheet having a zinc-based plating on its surface was used as the base material. The zinc content of the plating and the average thickness of the plating are shown in Table 1.

[0060] (Production of welded joint) Two base materials (steel plates) were used and gas-shielded arc welding was performed in the joint format shown in Table 1. When producing a welded joint by arc welding, the first steel plate and the second steel plate were welded under the following welding conditions, for example: welding current: 200 A, welding voltage: 22.4 V, welding speed: 100 cm / min, welding gas: 20% CO ２ +Ar, gas flow rate: 20 L / min Welding wire: YGW16, manufactured by Nippon Steel Welding Industry Co., Ltd., φ1.2 mm (C: 0.1 mass%, Si: 0.80 mass%, Mn: 1.5 mass%, P: 0.015 mass%, S: 0.008 mass%, Cu: 0.36 mass%) Welding torch tilt angle: 45°

[0061] At that time, in Examples No. 1 to 19 and No. 22, welding was performed while spraying the type of fluid shown in Table 1 onto the back surface of the weld at the flow rate shown in Table 1.

[0062] The ZnO compound attached along the heat-affected zone on the back side of the weld was confirmed by the above-mentioned method, and the length L of the ZnO compound in the welding direction was measured. Ｚ１ and L Ｚ２ and the length L of the weld metal in the welding direction Ｗ The results, as well as the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction (ZnO length ratio), are shown in Table 1. In Table 1, deviations from the requirements of the present disclosure are underlined.

[0063]

[0064] As shown in Table 1, in Examples (Nos. 1 to 19) in which welding was performed while spraying fluid within the aforementioned flow rate range, the length of ZnO compounds in the welding direction was reduced. This is believed to have resulted in excellent surface coating film formation and enhanced corrosion resistance. In No. 20 (Comparative Example), the zinc content of the zinc-based plating layer was low to begin with, which is believed to have suppressed the generation of ZnO compounds. In No. 21 (Comparative Example), the zinc-based plating layer was thin, resulting in a small amount of zinc evaporation, which is believed to have suppressed the generation of ZnO compounds. In No. 22 (Comparative Example), the zinc-based plating layer was too thick, resulting in a large amount of zinc evaporation. This is believed to have prevented the generation of ZnO compounds, even when welding was performed while spraying fluid. This is believed to have resulted in poor surface coating film formation and poor corrosion resistance. In No. 23 (Comparative Example), welding was performed without spraying fluid, resulting in a large amount of ZnO compound adhesion. Therefore, it is thought that the ability to form a coating film on the surface is poor, and the corrosion resistance is also poor.

[0065] The disclosure of Japanese Patent Application No. 2023-223069 is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

[0066] 2 Base material, 3A, 3B, 3C, 3D Weld metal, 4, 4A, 4B, 4C, 4D Heat-affected zone, 6, 60, 6A, 6B, 6CA, 6CB ZnO compounds, 8 Zinc-based plating layer, 10, 100 Weld joint, 21A, 21B, 21C, 21D, 22A, 22B, 22C, 22D Steel plate, 30 Nozzle, Y Weld back surface, Z Fluid

Claims

1. A welded joint formed by overlapping and welding two or more steel materials, having a weld metal, a heat-affected zone, and a steel material part, wherein the steel material part has a zinc-based plating layer containing 85% by mass or more of zinc and having an average thickness of 3 μm or more and 30 μm or less on its surface, ZnO compound adheres along the heat-affected zone on the back side of the weld, and the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction of the weld metal is more than 0% and 80% or less.

2. The welded joint according to claim 1, wherein the ratio of the length of the ZnO compound in the welding direction to the length of the heat-affected zone in the welding direction of the weld metal is 10% or more and 60% or less.

3. The welded joint according to claim 1, having the ZnO compound, a chemical conversion coating, and an electrodeposition coating at least on the surface of the heat-affected zone.

4. A welding process for manufacturing a welded joint by overlapping and welding two or more steel materials having a zinc-based plating layer containing 85% by mass or more of zinc and having an average thickness of 3 μm or more and 30 μm or less, wherein in the welding process, while blowing at least one fluid selected from the group consisting of a gas and a liquid at a flow rate of 0.3 m / s or more and 14.0 m / s or less onto the portion that becomes the heat-affected zone on the back side of the weld, the welding is performed.

5. The method for manufacturing a welded joint according to claim 4, wherein the fluid is an inert gas.

6. An automotive part having the welded joint according to any one of claims 1 to 3.

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